Titanium oxide can convert light energy into chemical energy or electrical energy. Therefore, titanium oxide is expected to be applied to photocatalysts that decompose various toxic substances, and to dye-sensitized solar cells that have been attracting attention as next-generation solar cells.
Titanium oxide has three types of crystal structures: rutile, brookite, and anatase forms. Among these three forms, anatase-type titanium oxide is known to have excellent photocatalytic characteristics and excellent photoelectric transfer characteristics for dye-sensitized solar cells.
A method for producing a titanium oxide film on the surface of metallic titanium or titanium alloy (an alloy comprising titanium as a main component) is conventionally known. Another known method is to anodize metallic titanium or titanium alloy in a dilute solution of an inorganic acid (e.g., phosphoric acid) that does not have an etching effect on titanium.
However, these methods only result in the formation of amorphous titanium oxide, which does not have a crystal structure. Amorphous titanium oxide is known to have no photocatalytic characteristics and no photoelectric transfer characteristics for dye-sensitized solar cells.
Moreover, a method for producing an anatase-type titanium oxide film by anodization has been considered.
PTL 1 proposes a method comprising anodizing a titanium plate in a dilute acid solution of phosphoric acid or the like that does not have an etching effect on titanium, and then heating the plate under an oxidizing atmosphere.
PTL 2 proposes a method comprising anodizing titanium immersed in an electrolytic bath comprising sulfuric acid or the like that has an etching effect on titanium, and then further anodizing the titanium after adding ammonium fluoride to the electrolytic bath.
PTL 3 proposes a method comprising forming titanium nitride on the surface of titanium, then immersing the titanium in an electrolyte solution containing an inorganic acid or the like that has an etching effect on titanium, and performing anodization by applying a voltage higher than the spark discharge generating voltage.
Furthermore, the production of frictional sliding members having wear resistance using titanium nitride has been considered.
PTL 4 proposes a method for producing highly oriented titanium nitride, the method comprising ionizing metallic titanium by arc discharge, and reacting the metallic titanium with nitrogen gas, thereby forming a crystalline thin film of titanium nitride on the surface of a substrate.
That is, only the following methods were known as prior art: a method comprising forming titanium nitride on the surface of titanium, and then performing anodization by applying a voltage higher than the spark discharge generating voltage in an electrolyte solution containing an inorganic acid or the like that has an etching effect on titanium; and
a method comprising anodizing a titanium plate in a dilute acid solution of phosphoric acid or the like that does not have an etching effect on titanium, and then heating the plate under an oxidizing atmosphere.
However, these methods had the following problems. They were impractical due to their complicated processes; strong acid having a high risk of having an etching effect on titanium was used in anodization; high current and high voltage were required; and their handling was complicated because harmful gas and mist were generated during handling.